IN THE FOOTSTEPS OF JIM THOMPSON: PETROLOGY OF AMPHIBOLE-BEARING ASSEMBLAGES FROM CENTRAL MASSACHUSETTS AND SW NEW HAMPSHIRE

Jim Thompson’s portrayal of amphibole structure as alternating (010) layers with pyroxene- and mica-like structures (Thompson, 1970) simplified our understanding of amphibole crystal chemistry and gave new meaning to analogous petrological behavior among mica, amphibole, and pyroxene (biopyriboles). In central Massachusetts and southwestern New Hampshire, Peter Robinson, Howard Jaffe, and coworkers were among the earliest researchers pursuing the petrology of amphibole- and multiple-amphibole-bearing rocks. Reports of assemblages with multiple, coexisting amphiboles increased as more researchers learned to recognize and anticipate these minerals in the field. This general phenomenon was summarized succinctly by Jim Thompson (1964): “An outcrop…may prove a treasure house of discovery when revisited months or years later and seen again through enlightened eyes. It is simply that we rarely find more than what we are looking for or expect.”

After my graduate work at the University of Massachusetts, Amherst, geochronology greatly “enlightened” our understanding of metamorphism and deformation in southern New England. Three metamorphic episodes (Acadian, 408-390 Ma; Quaboagian, 370-350 Ma; Northfieldian, 300-285 Ma) replace the single Acadian metamorphism formerly thought to affect the area. The boundary separating areas dominated by Northfieldian and Quaboagian metamorphism runs through the Bronson Hill Anticlinorium (BHA). The succession of metamorphic and deformational events results in partial to complete metamorphic overprints. Orthoamphibole (OAM)-bearing rocks, including outcrops visited by Jim Thompson, are among the best both to preserve Quaboagian peak metamorphic assemblages and record partial Northfieldian recrystallization. Quartz-bearing OAM gneisses commonly contain plagioclase ± aluminosilicate ± staurolite (ST) ± cordierite (CRD) ± garnet (GRT) ± chlorite (CHL). Textures in these gneisses suggest peak Quaboagian assemblages were affected by or partially recrystallized at Northfieldian conditions. The textures include partial replacement of coarse OAM + CRD by CHL + ST + kyanite and a second generation of compositionally distinct OAM that in some samples may be controlled by the OAM solvus, which suggests peak Northfieldian temperatures under about 600°C.